The present invention relates to an apparatus and method for level measurement and more particularly to a two-axis inclinometer and an integrated network of inclinometers capable of being centrally controlled and monitored.
It is often desirable to measure the inclination of an object relative to one or more horizontal axes. A number of devices directed to this general purpose have been described.
A spirit level includes an enclosed tube that is partially filled with a liquid and has a gas bubble filling the remaining volume inside the tube. In a standard spirit level, a glass tube houses the liquid and is either slightly curved with its convex side upward or is ground with a curved inner bore. The glass tube is supported on a rigid base. Naturally, the gas bubble tends to seek the highest point in the tube containing the liquid and always comes to equilibrium at the same position whenever the base has the same slope with respect to the horizontal plane. Therefore, the user can measure inclination by observing the position of the bubble. However, there are obvious drawbacks to the spirit level, including its lack of precision as a result of its reliance on visual inspection of the bubble position.
A number of devices generally referred to as inclinometers or clinometers measure the magnitude and direction of inclination and produce an electrical signal as a function of inclination angle. Thus, an inclinometer can hold a dielectric or a conductive fluid in a chamber having two or more electrodes on or near opposite walls of the chamber, the electrodes being partially covered with fluid and with the fluid contact area on each electrode being dependent on the inclination angle of the chamber relative to gravity.
In a resistive inclinometer, the fluid has a specific conductivity and the resistance between the two electrodes varies with the contact area between the fluid and the electrodes. In a capacitive inclinometer, a dielectric fluid is used in the chamber. The electrical capacitance difference between the electrodes varies with the electrode-fluid contact area according to the inclination angle of the chamber. A common electrode can be provided such that capacitance of the common electrode is measured against that of each of the measuring electrodes.
Some prior art inclinometers have been described as having the various objectives of improving the utility, precision, accuracy, durability, efficiency or affordability of an inclinometer.
U.S. Pat. No. 4,167,818 to Cantarella et al. describes a single axis electronic inclination gauge fixed within a straight bar and having a digital output. A gravity dependent potentiometer cell operates in conjunction with a balancing bridge circuit to provide an analog voltage output whose magnitude is a function of inclination angle. This output is converted to digital values by a standard A/D converter. The cell is a sealed horizontally oriented cylindrical chamber partially filled with a semi-conductive fluid such as an alcohol. The chamber contains a central circular electrode opposing four symmetrically placed arcuate peripheral electrodes. This structure allows operation of the gauge from either of two orthogonal starting orientations. In operation, the center electrode has a function analogous to the slider of a variable resistor whereby its effective position in the resistance path between a pair of adjacent electrodes in circuit depends on the inclination angle of the chamber.
U.S. Pat. No. 4,811,491 to Phillips et al. describes an electronic capacitive inclinometer capable of measuring inclination in two axes. A non-conductive vertically oriented hollow outer cylinder has two pairs of symmetrically placed electrodes on its outside surface. An inner cylinder is spaced from the inner walls of the outer cylinder and functions as a common grounded electrode. A dielectric fluid is provided in the gap between the two cylinders. A pair of current sources charges each electrode of an opposing pair relative to the common electrode.
Analog output signals are generated as a function of the differential change in capacitance between,opposing electrodes. Their outputs are coupled to an amplifier by means of an RC low pass filter circuit. A clock-driven switch driver switches operation between the orthogonal pairs of electrodes.
U.S. Pat. No. 4,528,760 to Plummer describes a single-axis inclinometer having a common grounded electrode plate on one flat vertical outer face of a glass or plastic horizontally oriented cylinder housing and three symmetrically placed arcuate segment electrodes on an opposite outer face. A dielectric fluid partially fills the housing such that its level is always completely above one of the segment electrodes, thereby allowing inclination measurements for up to 360 degrees of inclination. Inclination is measured about a horizontal axis running through the cylinder outer surfaces. Each segment electrode is connected to a different oscillator circuit supplying a frequency to a terminal corresponding to the capacitance between the segment electrode and the common electrode. Comparison of frequency signals between terminals provides signals indicative of capacitance difference and a decode circuit is said to produce a corresponding,digital signal.
xe2x80x9cSome inclinometers have sought-to avoid or compensate for the effects of temperature variations and electrochemical degradation. In a capacitive inclinometer, for example, temperature changes affect the dielectric constant of the sensor fluid. The cost of existing precision inclinometers, however, has been prohibitive. German Application No. DE 197 25 248 published Dec. 17, 1998 describes a two-axis capacitance inclinometer sensor provided with means for compensating for temperature changes in the environment of the sensor. The electrodes of the sensor area coated with a dielectric material such as TEFLON(copyright) polymer with the purpose of reducing screening by the sensor fluid to increase sensitivity and to improve efficiency. Temperature compensation is achieved by selecting a sensor fluid having a high dielectric constant and by including a comparison circuit wherein sensor data are adjusted based on temperature data.xe2x80x9d
U.S. Pat. No. 3,992,951 to Erspamer et al. describes an analog resistive accelerometer having a horizontally oriented cylindrical housing and including a reference electrode to compensate for changes in electrolytic fluid temperature. The reference electrode is completely immersed at all times and therefore introduces a temperature-only dependent parameter to a signal amplifier in order to cancel the effects of temperature on resistivity measurements. A different compensation circuit is provided across the amplifier to detect any net DC voltage in the electrolytic fluid as a result of the reference circuit and to supply a compensating voltage to eliminate it and prevent polarization of the fluid.
xe2x80x9cU.S. Pat. No. 4,912,662 to Butler et al. describes a capacitive inclinometer having aligned opposing sensor plates including corresponding conducting sectors. Pairs of opposing sectors from variable capacitors. A conductive grounded peripheral edge surrounds the plates to form a fluid cavity therebetween containing a conductive fluid. The sensor plates are preferably coated with a dielectric such as TEFLON(copyright) polymer. It is said that the dielectric coating effectively reduces the distance between the capacitive plates creating a high capacitance sensor without the manufacturing difficulty of actually placing the plates in close proximity to each other. Alternatively, the fluid is a dielectric and the plate coating is eliminated. The variable capacitors are placed in an oscillator circuit that has a frequency/period output in accordance with the capacitance of the capacitors. A microchip and memory are provided with a stored look-up table which is calibrated to the individual sensor for converting the output to units of inclination angle. It is said that the interface between the sensor and the microprocessor does not require an A/D converter. A temperature normalizing circuit on the microprocessor stores reference values of capacitance for comparison to measured values.xe2x80x9d
U.S. Pat. No. 5,428,902 to Cheah describes a two-axis resistive inclinometer provided with a Schmitt trigger circuit for producing a frequency measurement of inclination angle to be sent to a microprocessor. Counter circuitry is used to determine the frequency. A sensor includes a vertically oriented cylindrical vessel containing an electrolytic fluid and five electrode pins immersed in the fluid to allow resistance measurements between any two electrodes across the Schmitt trigger. The sensor is thus made the variable resistance element in the Schmitt circuit. Electrode polarity is periodically reversed to minimize net DC current flow through the electrodes which can change the resistivity of the electrolytic fluid. The Schmitt trigger circuit is cycled between the electrodes to provide the necessary tilt measurements.
These and other devices have not achieved a satisfactory combination of the often aspirational objectives of an inclinometer. It would be desirable to have an electronic inclinometer combining, among other things, two-axis measurement, precision, reliability, simplicity, low power consumption and low cost.
It has further been proposed to arrange a plurality of inclinometers in a centrally monitored network. Such a network is desirable in several applications where inclinometer data is taken. For example, structures such as buildings, runways, machinery, mining facilities, pipelines and the like may require comprehensive monitoring at numerous locations of forces impacting the structure for purposes of safety and/or maintenance. Central monitoring adds convenience and safety to the process of data collection.
U.S. Pat. No. 4,831,558 to Shoup et al. describes a linear network of transducer units linked in a linear array to a central controller, each unit having a microprocessor and buffer circuitry. The transducers include a detector of a physical parameter. A microchip associated with each unit provides a programmable address and the capability of filtering and converting a transducer signal into a digital format, temporarily storing the digital data and transmitting it to the controller. The buffer circuit, controlled by the microprocessor, directs incoming and outgoing data between the controller and the transducer. Transducer data is carried by a system bus through an interface to the central controller. The controller is capable of polling data from individual units in real time or on a timed schedule. Each transducer microchip is capable of receiving a uniquely addressed polling signal from the central controller. The transducer units can be either centrally powered, preferably by a 5 volt line, or alternatively can be individually powered.
Unfortunately, the network of the ""558 patent has several drawbacks making it less than ideal for practical industrial applications. For example, the power supply cannot accommodate the long branched chains of inclinometer units desirable for large scale applications. Further, a constant power supply is required to maintain the individual memories of the units. Large amounts of power are also consumed by its conventional A/D converter. The network is further limited in terms of versatility due to its inability to operate in an autonomous mode wherein power is automatically switched on periodically and measurements are taken and stored in each unit. The addressing capability of the units further does not include a built-in permanent logical address which can be integrated with an individual calibration file.
German Application No. DE 197 25 248 (also referred to above) discloses a centrally monitored inclinometer network. A capacitive inclinometer includes a sensor and a built-in microprocessor having an integrated memory. The microprocessor controls the sensor according to a preset time program. Capacitance is measured by a frequency generator followed by A/D conversion and storage of digital data in memory. A built-in interface provides for transmission of data to a central controller. The sensors are provided as a network in a parallel or serial array. Measurement can be conducted on demand by an operator or periodically by program. Data polling from the sensors can be conducted in real time or from inclinometer memory. Unfortunately, this network does not meet current needs. The microprocessors require continuous power to record and store only limited amounts of data and a power shut-down causes a complete loss of data. Overall power consumption is high because of continuous consumption and the A/D conversion device employed. Further, the network is limited by its capacity for data storage and transmission.
Therefore, it is inflexible and cannot easily accommodate additional transducers for measuring physical phenomena other than inclination.
Among other drawbacks, previously existing systems lack adequate data processing and presentation means. It would therefore be desirable to have a rugged, durable, efficient and inexpensive network of centrally controlled precision two-axis inclinometers without the shortcomings of previously existing devices.
Accordingly, it is a primary object of the present invention to provide an improved network of centrally monitored electronic inclinometers meeting the criteria set forth above and others.
It is another object of the invention to provide a resistive or capacitive inclinometer cell combining low cost and precision.
It is another object of the invention to provide an inclinometer having reduced power consumption.
It is another object of the present invention to provide an inclinometer having an improved temperature compensation feature.
It is another object of the present invention to provide an inclinometer having improved drift correction.
It is another object of the invention to provide an electronic inclinometer having simple and inexpensive circuitry and construction.
It is another object of the invention to provide an electronic inclinometer that produces a frequency signal as a measure of inclination angle, avoiding the need for conventional A/D conversion during signal processing.
It is another object of the invention to provide an electronic inclinometer having a solid state memory and a built-in permanent logical address.
It is another object of the present invention to provide an inclinometer having a power saving autonomous or sleep mode and a memory capable of storing data without a constant power supply.
It is further an object of the present invention to provide a centrally controlled inclinometer network including logical branches of inclinometer units.
It is another object of the invention to provide improved data storage and processing means for an inclinometer and network including software for a central controlling means.
It is another object of the present invention to provide an inclinometer network including a master device to centrally control the power supply to a network in a programmable autonomous operation mode.
Briefly, these objects and others are attained by an inclinometer and a centrally controlled and monitored network of said inclinometers. The inclinometer embodiments described in detail herein have capacitive sensors, however it is to be understood that the sensor cell of the invention can be either resistive or capacitive.
In one aspect of the invention, an inclinometer sensor cell is provided for measuring inclination in two orthogonal axes.
In one embodiment, a sensor includes a metal housing which can be a vertically oriented cylinder and a plurality of measuring electrode pins partially immersed in a dielectric fluid contained within the cell. Preferably, four electrodes are symmetrically placed inside the housing near its perimeter. The sensor preferably further includes reference electrodes to provide signals compensating for the effects of temperature and electrochemical changes in the sensor cell. The reference electrodes are immersed in the fluid near the center of the housing making their outputs independent of inclination. The reference electrodes can be coaxial with an outer electrode having a plurality of apertures. One benefit of the reference electrodes is that the measuring electrodes need not comprise costly precious metals and need not be coated with a dielectric material. Additionally, the fluid provided in the cell need not have a particularly high dielectric constant. In this manner precision and accuracy are achieved at a low cost.
In another embodiment of the invention, a sensor cell comprises a reservoir defined by an insulating glass or plastic housing containing a dielectric fluid. Electrodes are provided as opposing vertical metal strips applied to opposite sides of the housing exterior, isolated from the fluid. Alternatively, the housing can be provided with symmetrically arranged dielectric pipes containing the fluid and having the electrodes disposed thereon. Linearity of response can be enhanced by employing a housing having spherical outer surfaces.
Each inclinometer in the network of the invention further includes an electronic block. The electronic block includes a microprocessor capable of receiving and managing signals from the sensor cell and from a central computer. In a preferred embodiment, each sensor electrode is linked to an RC frequency generator. When measurements are taken, the RC generator measures the capacitance between the electrode and a common electrode or ground and emits a frequency as a function thereof. The microprocessor receives the frequency signals of all of the RC generators and, acting as a counter, counts the differences between the frequencies corresponding to opposing electrodes. One major advantage of this configuration is significantly reduced power consumption due to the absence of a conventional A/D converter. The microprocessor further includes a unique built-in permanent logical address. This permits the construction of a network of several programmable logical inclinometer branches, as discussed in greater detail hereinafter. A standard serial interface is provided for connection of the inclinometer to the network.
The inclinometer of the invention can include a temperature drift compensation circuit or a built-in temperature gauge providing data for a temperature compensation function. A calibration file specific to each particular sensor can also be provided.
The inclinometer of the invention is provided with memory means, preferably a solid state memory externally linked to the microprocessor as part of the electronic block. A constant power supply is therefore not required to maintain data stored in the external memory.
In another aspect of the invention, a central controlling means such as a computer is linked to and capable of remotely programming and operating an inclinometer network via standard serial interfaces. The controlling means allows operation of a network via any known communication means such as telephone, internet, or hard wire. The network can be operated and monitored in real time and additionally is programmable to periodically poll data in an autonomous mode. The autonomous mode includes a sleep phase in which power to the network is completely shut down. In this connection, a central master device is preferably provided for periodically switching on power to the network only at specified times of measurement taking.
The controlling means can be equipped with specialized data manipulation software for displaying network data in useful and convenient forms. For example, network data can be comprehensively displayed by a computer in a 3D format which visually represents conditions in a structure such as a building, runway or the like.
The invention will be more fully understood with reference to the drawings and the following description of the presently preferred embodiments.